This invention relates to an electroviscous liquid which undergoes an increase in viscosity on application of a voltage.
Electroviscous liquids (EVLs) are dispersions of fine-particle solids in hydrophobic and electrically non-conductive oils of which the viscosity may be increased very quickly and reversibly from the liquid to the plastic or solid state under the effect of a sufficiently strong electrical field. Their viscosity responds both to electrical d.c. fields and to a.c. fields, the current flowing through the EVL having to be extremely low. Accordingly, EVLs may be used for any applications in which it is desired to control the transmission of powerful forces by low electric power levels, for example in clutches, hydraulic valves, shock absorbers, vibrators or systems for positioning and holding workpieces in position.
In addition to the requirements which an EVL generally has to satisfy, such as a good electroviscous effect, high temperature stability and chemical stability, the abrasiveness and sedimentation stability behavior of the disperse phase play an important part in practical application. Ideally, the disperse phase should not sediment, but at all events should be readily redispersible and should not cause any abrasion under extreme mechanical stressing.
In qualitative terms, the increase in viscosity which an EVL undergoes on application of an electrical field may be explained as follows: the colloidally stable disperse particles polarize in the electrical field and agglomerate through dipole interaction in the direction of the field, resulting in the increase in viscosity. The agglomeration is reversible: if the electrical field is switched off, the particles redisperse and viscosity is reduced to the original value. The polarizability of the disperse phase is thus an important requirement for the development of an electroviscous effect. For this reason, ionically or electronically conductive materials are often used as the disperse phase.
In some of the EVLs corresponding to the prior art, the disperse phase consists of organic solids, such as for example saccharides (DE 2 530 694), starch (EP 2 842 268 A2, U.S. Pat. No. 3,970,573), polymers (EP 150 994 A1, DE 3 310 959 A1, GB 1,570,234, U.S. Pat. No. 4,129,513, ion exchanger resins (JP 92 278/975, JP 31 221/1985, U.S. Pat. No. 3,047,507) or silicone resins (DE 3,912 888 A1). However, inorganic materials have also been used, including for example Li hydrazine sulfate (U.S. Pat. No. 4,772,470 A), zeolites (EP 265 252 A2), silica gel (DE 3 517 281 A1, DE 3 427 499 A1) and aluminium silicates (DE 3 536 934 A1).
In the case of the substances mentioned, the electroviscous effect is attributable to the charging of the solids with water. Small water contents increase ionic conductivity and hence the polarizability of the disperse particles which is essential to the development of the effect. However, water-containing systems show poor chemical stability. In addition, the temperature range in which these liquids can be used is limited.
In the case of other electroviscous liquids, efforts have been made to overcome the disadvantages mentioned by replacing the water-containing disperse phase with a substantially water-free, electronically conductive phase which consists of partly coated, finely disperse metals, such as for example aluminium (JP 016 093, JP 0117 2496), or dielectrics, such as for example TiO.sub.2 (SU 715 596), CaTiO.sub.3 or BaTiO.sub.3 (JP 53/17585), hydrolyzates of metal alkoxides (EP 341 737) or hollow glass bodies (J 0117 2496). However, due to the hardness of the dispersed particles, the described EVLs are abrasive and, hence, are of only limited use for practical applications involving high shear stressing. Carbon-black-filled bead polymers (JP 016 093) or conductive polymers, such as for example polypyrrole or polyacetylene (JP 0126 0710), have also been discussed as substitutes for the water-containing phase.
With the water-containing systems, the optimal properties of the disperse phase can be effectively established by variation of the water content or by modification of the solid matrix. Thus, DE 2 802 494 C2 describes an improvement in the electroviscous effect by introduction of free or neutralized acid groups into a water-containing polymeric phase. In the production of EVLs based on electronically conductive disperse phases, the dispersion particles often had to be aftertreated because of the high electrical conductivity of the starting materials. Thus, JP 016 093 describes the passivation of carbon-black-filled bead polymers by subsequent coating of the polymer particles with polyvinylidene fluoride. However, production costs are greatly increased by aftertreatments of the type in question.
The above-mentioned EVLs corresponding to the prior art are generally produced by dispersion of a solid in a dispersion medium, such as for example halogen-free or halogenated hydrocarbons, aromatic hydrocarbons or silicone oil. The viscosity of the suspension formed depends upon the shape and size or size distribution of the dispersed particles and upon the solids concentration and dispersion effect of any dispersion aids used. High volume-related solids contents for low viscosities are difficult to achieve where non-spherical particles are used.
The problem addressed by the present invention was to provide a water-free, non-abrasive, non-sedimenting EVL having good electroviscous properties which would be distinguished by a low basic viscosity despite a high content by volume of disperse phase.